The addition of parenteral fluids to the blood streams of patients by means of cannulas inserted into the veins has been conventional for many years. More recently, parenteral systems have been introduced in which the fluids are positively pumped into the patient's veins, as opposed to earlier systems in which gravity was the motivating force. Pumping has become prevalent because it permits more precise control of fluid flow. Still more recently, systems have been developed in which the pressure in the line connecting the pump to the vein is monitored and a pressure signal is transmitted to a microprocessor or similar device for continuous monitoring of the pressure, so as to determine whether the pressure is within certain limits indicative of proper fluid flow. The goal of such instruments is to provide an indication of whether the cannula is properly in the vein, whether the vein flow is normal, and so on. See, for example, U.S. Pat. No. 4,460,355 to Layman, in which a further problem is addressed, that of distinguishing spikes in the fluid pressure due to the discontinuous action of the pump from pressure changes due to abnormal conditions which should be detected so as to give an alarm. The Layman patent provides means for only measuring the pressure of the pumping cycle during certain periods when pressure spikes caused by the pump action may be presumed not to be present.
The applicant is also aware of U.S. Pat. No. 4,534,756, assigned to the present assignee, entitled "Fault Detection Apparatus And Method For Parenteral Infusion System." This application broadly discloses a system in which the pressure waveform generated by a pump which produces a pulsating pressure waveform is monitored from cycle to cycle. When a change in the waveform out of preset limits is detected, an alarm is given. While this system was a substantial advance, it frequently could not distinguish fluid flow faults from normal pressure variations particularly at high flow rates.
Accordingly, the art requires an improved intravenous flow monitor which is capable of distinguishing between actual problems, e.g. occlusion of the vein, a cannula becoming detached from the patient's arm, or penetrating through or being withdrawn from the vein so as to be disposed in the surrounding soft tissue, and between artifacts such as relative changes in elevation of the pump and the point of injection of the fluid into the patient.